The present invention relates to a charging device having a charging member which is disposed opposite to the surface of a movable charged body, and charges the charged body by applying a voltage obtained by superposing an AC voltage on a DC voltage to the charging member to generate electric discharge between the charging member and the surface of the charged body. This invention also relates to an image formation apparatus having the charging device.
The above-mentioned type of charging device has conventionally been used widely in various types of machine and device. For example, the charging device is applied in an image forming device formed as a digital copier, a printer, a facsimile, or a multifunction machine provided with at least two of these functions. In order to form an electrostatic latent image on a charged body composed of an image carrier, this charging device charges the image carrier.
When such a charging device applies charge to the charged body, it is important that the surface of the charged body is uniformly charged without charge unevenness occurring on the surface of the charged body, that is, it is important to increase charge uniformity on the surface of the charged body. This will be explained by referring to the charging device used in the image forming device. The image carrier charged by the charging device is exposed to form an electrostatic latent image on the image carrier, and this electrostatic latent image is visualized as a toner image by a developing device. However, if there is charge unevenness on the surface of the charged body when the image carrier is charged by the charging device, density unevenness occurs on the developed toner image, so that the quality of the obtained image is degraded.
Conventionally, various types of configuration have been proposed in order to solve these problems. One of the configurations is a known charging device in which a peak-to-peak voltage of an AC voltage to be applied to a charging member is set to a value which is twice or more a charge start voltage (Japanese Patent Application Laid-Open No. 63-149669). The charge start voltage mentioned here is an absolute value of such a voltage at the instant when a charged body starts to be charged through application of only a DC voltage to the charging member and gradual increase of the absolute value of the applied voltage.
This type of charging device can more effectively enhance the charge uniformity of the charged body as compared to charging devices according to another proposals. However, the inventors have found, after careful studies on the charging device according to this proposal, that the material of the charging member capable of uniformly charging the charged body is limited. For example, when the entire charging member made of metal is used, it is difficult to control a discharge current producing between the charging member and the charged body so as to uniformly charge the charged body, even if the voltage to be apply to this charging member is set to the value. Thus, it is impossible to sufficiently increase charge uniformity of the charged body.
Further, some of the conventional charging devices has the charging member whose electric resistance varies depending on environmental variations. Thereby the charge uniformity of the charged body is largely lowered.
It is object of this invention to provide a charging device that can keep charge uniformity of a charged body at a high level without being largely affected by a material of a charging member and even if its environment changes. Another object of this invention is to provide an image formation apparatus having such a charging device.
In order to achieve the former object of this invention, the charging device according to this invention is proposed as follows. That is, in the above-mentioned type of charging device, a frequency f(Hz) is set so as to satisfy f(Hz)xe2x89xa740(1/mm)xc2x7v(mm/sec) where the frequency of an AC voltage is f(Hz) and a movement rate of the surface of a charged body is v(mm/sec).
Further, it is advantageous that a peak-to-peak voltage of an AC voltage to be applied to the charging member is set to a voltage which is twice or more a charge start voltage of the charged body.
Further, in the charging device according to this invention, it is advantageous that the charging member is positioned in a non-contact state with respect to the surface of the charged body while charging the charged body.
Further, in the charging device according to this invention, it is advantageous that the charging member is formed with a material having JIS A hardness of 90 degrees or more.
Further, in the charging device according to this invention, it is advantageous that the charging member is made of metal.
Further, in the charging device according to this invention, it is advantageous that the charging member is positioned in contact with the surface of the charged body while charging the charged body.
Further, in the charging device according to this invention, it is advantageous that the charging member has an elastic body.
Further, in the charging device according to this invention, it is advantageous that the charging member comprises a base substance to which the voltage is applied, and an intermediate resistor which is provided on the side of the base substance facing the surface of the charged body and has a volume resistivity higher than that of the base substance.
Further, in the charging device according to this invention, it is advantageous that the charging member is cylindrically formed.
Further, in the charging device according to this invention, it is advantageous that the charging member is formed as a charge roller that rotates.
In order to achieve the latter object of this invention, an image formation apparatus as follows is proposed. This image formation apparatus has the charging device according to this invention, and a charged body is composed of an image carrier with a toner image formed on its surface.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.